The invention relates to the field of methods of communicating data between an airborne platform and ground stations, particularly but not exclusively for air traffic control.
The context of the invention is described with reference to FIG. 1, which shows one of the problems in the present state of the art.
In this figure, there can be seen a communications device CMU0 of the type to be found on board airborne platforms and suitable for communicating at a given instant with a ground station VGS1, VGS2, or VGS3 via a bidirectional communications module VDR configured to operate on the frequency of that station, using the very high frequency (VHF) data link (VDL) mode 2 protocol.
It should be recalled that the VDL mode 2 data communication protocol may be used in particular for air traffic control (ATC). The invention is situated in particular in the general field known as controller-pilot data link communications (CPDLC).
In known manner, the communications device CMU0 sends downlink messages referenced F1, F3 to the ground station VGS1, and it receives from this station uplink messages referenced F2, F4 in compliance with this protocol, e.g. for air traffic control.
In known manner, in accordance with this protocol, the ground stations VGS1, VGS2, VGS3 are suitable for broadcasting a message GSIF regularly, about once every 90 seconds, which message GSIF comprises at least the identifier of the transmitting station and possibly also a list LST of pairs in which each pair associates a frequency and the identifier of a ground station suitable for communicating on that frequency.
In the example of FIG. 1, the communications device CMU0 is thus regularly informed by the ground station VGS1 on frequency FQ1 that another ground station VGS3 exists and that that other station communicates on the frequency FQ2. It is also assumed that there exists a ground station VGS2 operating on the frequency FQ2.
However, so long as the communications device CMU0 is in communication with the ground station VGS1 on the frequency FQ1, it does not hear messages broadcast by the stations on the other frequencies.
FIG. 2 is in the form of a flow chart showing communication being set up, and being lost, and also attempts at reestablishing communication in the present state of the art.
The first step E10 in the flow chart shows communication being set up by the communications device CMU0 with the ground station VGS1 on the frequency FQ1.
Once communication is established, the communications device CMU0 acts in a step E70 to ensure that this communication is maintained.
References F5a to F5f in FIG. 1 show the behavior of the communications device CMU0 in compliance with the VDL mode 2 protocol on losing communication with the ground station VGS1.
In compliance with the protocol, the communications device CMU0 acts during the step E70 to perform six attempts prior to considering that communication has been disrupted, occupying a total duration of about 45 seconds.
When it is confirmed that communication has failed, the communications device deploys four strategies to attempt to reestablish communication during steps E40 (strategy 1), E50 (strategies 2 and 3), and E60 (strategy 4).
More precisely, during a step E40, the communications device attempts to set up communication with the ground stations heard on the current frequency FQ1.
In the event of failure, the communications device then attempts in a step E50 to set up communication on a frequency of the above-mentioned list LST, namely in this example the frequency FQ2. For this purpose, it configures (reference F6) the bidirectional communications module VDR so that it communicates on the frequency FQ2. Thereafter, the communications device attempts to set up communication with the ground station communicating on the frequency FQ2, namely in this example with the ground station VGS3 (reference F7).
If this attempt fails, i.e. after six repeated failures over a total duration of 45 seconds, the communications device attempts to set up communication (reference F8) with another ground station VGS2, previously heard on the same frequency FQ2 (reference F10).
If this attempt succeeds, the communications device CMU0 does indeed communicate with the ground station VGS2 on the frequency FQ2 as represented by the downlink and uplink messages referenced F11 and F9, and during the step E70, it ensures that this communication is maintained.
If during the step E50, all of the frequencies of the list LST have been tried without success, then the communications device CMU0 attempts during a step E60 to set up communication on a frequency that is predefined in the VDL mode 2 standard.
Finally, if this last attempt at setting up communication fails, the communications device CMU0 considers that communication in VDL mode 2 has been interrupted, and during a step E43 it switches over to another mode of communication, namely mode A in this example.
The present invention seeks to improve this communications method.